Correlations between the alpha angle and femoral head asphericity: Implications and recommendations for the diagnosis of cam femoroacetabular impingement

Objective To determine the strength of common radiographic and radial CT views for measuring true femoral head asphericity. Patients and methods In 15 patients with cam femoroacetabular impingement (FAI) and 15 controls, alpha angles were measured by two observers using radial CT (0°, 30°, 60°, 90°) and digitally reconstructed radiographs (DRRs) for the: anterior-posterior (AP), standing frog-leg lateral, 45° Dunn with neutral rotation, 45° Dunn with 40° external rotation, and cross-table lateral views. A DRR validation study was performed. Alpha angles were compared between groups. Maximum deviation from a sphere of each subject was obtained from a previous study. Alpha angles from each view were correlated with maximum deviation. Results There were no significant differences between alpha angles measured on radiographs and the corresponding DRRs (p = 0.72). Alpha angles were significantly greater in patients for all views (p - 0.002). Alpha angles from the 45- Dunn with 40- external rotation, cross-table lateral, and 60- radial views had the strongest correlations with maximum deviation (r = 0.831; r = 0.823; r = 0.808, respectively). The AP view had the weakest correlation (r = 0.358). Conclusion DRRs were a validated means to simulate hip radiographs. The 45- Dunn with 40- external rotation, cross-table lateral, and 60 radial views best visualized femoral asphericity. Although commonly used, the AP view did not visualize cam deformities well. Overall, the magnitude of the alpha angle may not be indicative of the size of the deformity. Thus, 3D reconstructions and measurements of asphericity could improve the diagnosis of cam FAI.